The equilibria between Al3 + and three thiocarboxylic acids (2-mercaptopropanoic acid (thiolactic acid (H2TLA), 3-mercaptopropanoic acid (H2MPA), 2-mercaptosuccinic acid (thiomalic acid (H3TMA)) were studied in NaCl aqueous solution at 0.15 ≤ I/mol L− 1 ≤ 1 and T = 298.15 K. The results showed the formation of ML and MLH species for all the investigated systems; in addition, MLOH and ML2OH have been observed for TLA, as well as ML2 and ML2OH for MPA, MLH2 and MLOH for TMA. For the ML species, in NaCl at I = 0.15 mol L− 1 and T = 298.15 K, the stability is fairly high, with logβML = 8.33, 8.756, 9.87 for TLA, MPA and TMA, respectively. The species formation at I = 0.15 mol L− 1 in NaCl was also investigated by using calorimetric, spectrophotometric and 1H NMR titrations. ΔH values of the main species, obtained by titration calorimetry, are endothermic, as typical for hard-hard interactions, where the contribution to the Gibbs energy of complexation is mainly entropic in nature. TΔS values referring to the ML species, at I = 0.15 mol L− 1 and T = 298.15 K, are 75, 53, 67 kJ mol− 1 for TLA, MPA and TMA, respectively. In the same conditions, for ML species, ΔH = 28, 3, 11 kJ mol− 1 for TLA, MPA and TMA, respectively. The speciation models and the formation constants of the main complex species obtained by the spectrophotometric and 1H NMR measurements at I = 0.15 mol L− 1 fully confirm those gained by potentiometry in the same conditions. The dependence of the stability on ionic strength was also analysed and the speciation profiles for all the systems under study were calculated at different ionic strengths. The sequestering ability of the thiocarboxylate ligands towards Al3 + was also evaluated in the same conditions. For example, at pH = 5, in NaCl at I = 0.15 mol L− 1 and T = 298.15 K, the sequestering power of thiocarboxylate towards Al3 + follows the trend TMA > TLA ≈ MPA.

The equilibria between Al3 + and three thiocarboxylic acids (2-mercaptopropanoic acid (thiolactic acid (H2TLA), 3-mercaptopropanoic acid (H2MPA), 2-mercaptosuccinic acid (thiomalic acid (H3TMA)) were studied in NaCl aqueous solution at 0.15 ≤ I/mol L− 1 ≤ 1 and T = 298.15 K. The results showed the formation of ML and MLH species for all the investigated systems; in addition, MLOH and ML2OH have been observed for TLA, as well as ML2 and ML2OH for MPA, MLH2 and MLOH for TMA. For the ML species, in NaCl at I = 0.15 mol L− 1 and T = 298.15 K, the stability is fairly high, with logβML = 8.33, 8.756, 9.87 for TLA, MPA and TMA, respectively. The species formation at I = 0.15 mol L− 1 in NaCl was also investigated by using calorimetric, spectrophotometric and 1H NMR titrations. ΔH values of the main species, obtained by titration calorimetry, are endothermic, as typical for hard-hard interactions, where the contribution to the Gibbs energy of complexation is mainly entropic in nature. TΔS values referring to the ML species, at I = 0.15 mol L− 1 and T = 298.15 K, are 75, 53, 67 kJ mol− 1 for TLA, MPA and TMA, respectively. In the same conditions, for ML species, ΔH = 28, 3, 11 kJ mol− 1 for TLA, MPA and TMA, respectively. The speciation models and the formation constants of the main complex species obtained by the spectrophotometric and 1H NMR measurements at I = 0.15 mol L− 1 fully confirm those gained by potentiometry in the same conditions. The dependence of the stability on ionic strength was also analysed and the speciation profiles for all the systems under study were calculated at different ionic strengths. The sequestering ability of the thiocarboxylate ligands towards Al3 + was also evaluated in the same conditions. For example, at pH = 5, in NaCl at I = 0.15 mol L− 1 and T = 298.15 K, the sequestering power of thiocarboxylate towards Al3 + follows the trend TMA > TLA ≈ MPA.